How Do You Translate a Latency Into a Percentile?

How do you translate something like a latency into a percentage to make it an SLO? A lot of that depends on what your telemetry looks like. It's got a ton of difference, mathematically, talking about aiming for 95% 120 second milliseconds or less versus the 95th percentile. We want to be 120 milliseconds or less, a very high percentage of the time.